Final Article in a 14-Part Series
Suction-Side System Design
Figure 1 shows most of the features discussed below.
Straight Run Into Suction
For satisfactory operation, a centrifugal pump requires a uniform velocity profile of fluid entering the impeller. This is more critical for pumps with high suction specific speeds and pumps with end suction nozzles. A swirling or distorted velocity profile can cause problems. Therefore, no short‑radius elbows (or tees) should be used near the pump. The pump's suction nozzle should be preceded by a straight run of pipe, about 10 pipe diameters in length. A longer straight section may be required if turbulence is severe or velocity is high (more than 10 ft/s).
The author is aware of one installation that successfully uses long-radius, tapered elbows at pump inlets, instead of straight runs. (A converging nozzle has a significant stabilizing effect on flowing fluid.)
Location and Size of Block Valve
The suction block valve should be line size. Since the line is almost always larger than the pump nozzle, the suction valve must be placed before the reducer.
Sloping of Suction Lines
Overhead suction lines should slope downward to the pump to allow draining of the suction system through the pump. Horizontal or below‑grade suction lines should slope upward toward the pump to prevent the collection of gas at any point in the line. A gas pocket increases the friction loss, and will sometimes cause gas to move as a slug into the pump. It is better to have free gas pass through the pump continuously, in small quantities.
Suction Strainer-Shape and Flow Area
Unless the suction system contains a permanent filter, a temporary conical strainer may be provided in the suction pipe for plant start‑up. The included angle of the cone should not exceed 45 deg, and the free flow area of the strainer should be at least three times that of the piping. This "witch's hat" should point upstream. Suction pressure must be closely monitored and recorded to determine if the strainer is becoming plugged. If suction pressure drops too low, the strainer must be removed and cleaned. If it cannot be ensured that a strainer will be kept clean, it should not be installed. A pump will often be damaged more severely by starving the suction than by passing solids.
Suction Reducer-Concentric and Eccentric
For horizontal‑inlet pumps, the reducer at the pump should be eccentric, with the straight side up. This avoids the creation of a high point in the piping where gas can collect. For top‑inlet pumps, the reducer may be concentric or eccentric, as required to obtain clearance with the discharge piping.
A gauge should be provided at the pump suction. Although a gauge connection in the pump nozzle is occasionally used, turbulence and non-uniform velocity distribution make this point less desirable. More accurate readings are obtained in the suction pipe, after a section of straight pipe. There should be no fitting or valve between the pump and gauge connection.
Vents should be provided in the suction piping and at the high point of the casing to allow priming the pump prior to start‑up.
Every centrifugal pump that can be operated too near shut‑off, should be provided with a minimum flow bypass. The bypass should be sized to pass the minimum flow that the pump manufacturer recommends or these articles have established. The bypass line should return to the suction vessel. It should not dump back into the suction line.
Because of the pulsing flow, the design of a suction system for a reciprocating pump requires more care than for a centrifugal pump. Improper design has often resulted in vibrating, noisy systems. Pulsations may be severe enough to damage instrumentation, system components and pump components.
The following lists provide system design guidelines (2), as illustrated in Figure 2:
A. The suction vessel should:
- Be large enough to provide sufficient retention time to allow entrained gas bubbles to rise to the liquid surface.
- Contain a weir plate to force gas bubbles toward the surface, with its top sufficiently below minimum tank level to avoid turbulence.
- Have feed and return lines enter below the minimum liquid level.
- Include a vortex breaker over the outlet (pump suction) line, or have sufficient submergence to prevent formation of a vortex.
B. The suction piping should:
- Be short and as direct as possible.
- Be one or two pipe sizes larger than the pump suction connection.
- Have average liquid velocity less than values from curves in Figure 3 (typically about 2 ft/s).
- Contain a minimum number of turns. Necessary turns should be accomplished with long‑radius elbows or laterals.
- Be designed to preclude the collection of vapor in the piping. (There should be no high points, unless vented.)
- Be designed so that NPSHa, allowing for acceleration head, exceeds NPSHR.
If acceleration head is excessive, include a suitable suction stabilizer, bottle or pulsation dampener located in the suction pipe, adjacent to the liquid end.